This invention relates to a method of precisely preloading a bearing onto a shaft. More specifically, this invention relates to a method of precisely preloading a spherical roller bearing onto a shaft.
Bearings are used for many different applications. Spherical roller bearings are bearings which contain an outer race, an inner race, and a plurality of freely rotatable spherical roller elements positioned between the two races. The inner race further contains a taper bore formed on the interior diameter thereof which is designed to mate with a taper machined into the shaft on which it is to be mounted. Spherical roller bearings are commonly used by the printing industry and by manufacturers employing rotary die cutters. Rotary die cutters can be used to cut many different kinds of materials. Normally, a pair of spherical roller bearings are used to rotatably mount a rotary die cutter onto a shaft which is then assembled into a die cutting module. When using such bearings, it is important that the bearings be precisely preloaded so as to provide a precision mounting of the rotary die cutter and also to extend the life of the bearing.
As stated above, the tapered bore on the internal diameter of a spherical roller bearing is designed to match up with a taper machined into the shaft onto which the bearing will be mounted. For those operations wherein a rotary die cutter is employed, a shaft will have the rotary die cutter mounted in the center thereof and will contain a pair of tapered surfaces, each located adjacent to one side of the die cutter. A shoulder is machined into the shaft adjacent to each tapered surface and a threaded portion is machined into the shaft adjacent to the tapered portions. A metal shim is slid onto the shalt and is positioned adjacent to the shoulder. The spherical roller bearings are then positioned onto the tapered surfaces. The thickness of each shim will dictate the distance an adjacent bearing will be able to move up the tapered surface. A lock nut is then threaded onto each end of the shaft and each lock nut will contact the inner race of one of the bearings. As the inner race of each bearing expands, it causes the bearing to be preloaded on the shaft. The lateral distance the bearing moves up the tapered surface is determined by the thickness of the metal shim which has been slid onto the shaft. If the bearing is not sufficiently preloaded, it will be sloppy and the shaft it is supporting will not revolve with the required precision. Conversely, if the bearing is excessively preloaded, it will fail prematurely requiring expensive downtime and replacement.
Today, many different methods are used to preload a bearing onto a shaft. Two common methods include the use of a bearing gauge and gauge blocks which are used to determine the proper shim thickness required to set the bearing preload. This method works well on brand new bearings but cannot be used to preload reconditioned bearings. A second method involves the use of a string which is wrapped around the outer race of a bearing after it has been secured in place by the lock nut. The free end of the string is attached to a spring scale, similar to those used to weigh fish. As the string is pulled, the resistance of the roller elements of the bearing can be felt. This method is very subjective to human error and therefore tends to be inaccurate, especially when performed by an inexperienced operator. Although this method does work on both new and reconditioned bearings, it will not work when the bearings are packed with certain synthetic greases which do not have an extreme pressure additive.
Now, a method has been invented for precisely preloading a new or reconditioned bearing onto a shaft. The method also works on bearings which have been packed with certain synthetic greases which do not have an extreme pressure additive.
Briefly, this invention relates to a method of precisely preloading a bearing onto a shaft, especially a spherical roller bearing. The apparatus includes a base plate and a support mounted to and extending upward from the base plate. The support is capable of supporting a shaft. The shaft has a shoulder, a tapered surface adjacent to the shoulder and a threaded portion adjacent to the tapered surface. A shim is positioned on the shaft adjacent to the shoulder and a bearing is positioned adjacent to the shim. The bearing has an inner race, an outer race and a plurality of roller elements positioned therebetween. The bearing is preloaded onto the shaft by a lock nut which is secured to the threaded portion of the shaft. The lock nut contacts the inner race of the bearing and forces the inner race to expand outwardly. This action causes the bearing to be preloaded onto the shaft. The apparatus further includes a motor mounted to the base plate. The motor has an output shaft which is axially aligned with and connectable to the outer race of the bearing. The motor is capable of rotating the outer race of the bearing at a predetermined, constant speed. A force sensor is attached to the motor which is capable of measuring the amount of torque required to rotate the outer race at the selected predetermined speed. The measured torque is then visually displayed on a display unit. When the measured torque is within a predetermined torque range, the bearing will be precisely loaded.
The method includes sliding a first shim onto the support shaft until it abuts the shoulder. The first shim has a predetermined thickness. A bearing is then slid onto the tapered surface and a lock nut is tightened onto the threaded portion of the shaft. As the lock nut contacts the inner race of the bearing it causes the inner race to expand. This reduces the distance between the inner and outer races and preloads the roller elements of the bearing. The outer race of the bearing is then connected to a motor and the motor is operated at a predetermined, constant speed. The torque required to rotate the outer race of the bearing is measured and compared to a predetermine torque range to determine if the bearing is precisely preloaded.
The general object of this invention is to provide a method of precisely preloading a bearing onto a shaft. A more specific object of this invention is to provide a method of precisely preloading a spherical roller bearing onto a shaft.
Another object of this invention is to provide a method of precisely preloading a new or reconditioned bearing onto a shaft.
Still another object of this invention is to provide a method of precisely preloading a bearing onto a shaft even when the bearing has been packed with certain synthetic greases which do not have an extreme pressure additive.
A further object of this invention to provide an easy and reliable method of precisely preloading a spherical roller bearing onto a shaft.
Still further, an object of this invention is to provide a relatively simple method for precisely preloading a spherical roller bearing onto a shaft.
Still further, another object of this invention is to provide a simple and straight forward method of precisely preloading a bearing onto a shaft which provides digital readouts of the measured torque in standard in-lb. units.
Other objects and advantages of the present invention will become more apparent to those skilled in the art in view of the following description and the accompanying drawings.